Aluminum metal-phosphate coating compositions for protecting metallic surfaces from oxidation and corrosion, particularly at high temperatures, are well known in the art. For example, U.S. Pat. No. 3,248,25 issued Apr. 26, 1966 to Charlotte Allen teaches a coating composition containing chromium and/or molybdenum to inhibit the reaction between an aqueous, acidic, phosphate component of the composition and a solid particulate metallic material, preferably aluminum powder.
Heretofore, phosphate-aluminum powder coating compositions for protecting metallic surfaces from oxidation and corrosion have been based on an acid-base reaction to neutralize the bonding solution and contain either hexavalent chromium or molybdenum to inhibit the oxidation of metallic aluminum. Both hexavalent chromium and molybdenum are considered toxic chemicals and are therefore environmentally disadvantageous. In particular, hexavalent chromium is rated as a carcinogen. Molybdenum is classified as a toxic heavy metal.
Sealing of metallic surfaces and protective coatings heretofore been difficult In particular, prior coatings have limited oxidation and corrosion resistance at temperatures at elevated temperatures.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a high-temperature oxidation and corrosion-resistant seal coating that does not require either chromates or molybdates to control reaction between the bonding solution and a metal product in the solution, e.g., aluminum. Furthermore, it is desirable to have such a seal coating that has improved resistance to oxidation and corrosion and is stable at elevated temperatures.